A known power seat apparatus for a vehicle disclosed in JP2000-85419A includes a pair of lower rails fixedly mounted on a vehicle floor on a left side and a right side respectively, and a pair of upper rails slidably supported by the pair of lower rails and supporting thereon a vehicle seat.
Each of the lower rails is provided with a nut member securely retaining therethrough the lower rail. Each of the upper rails is provided with a screw shaft rotatably supported and extending in a longitudinal direction of a vehicle, and with a drive mechanism for driving the screw shaft. The screw shafts are threadably engaged with the nut members. When the drive mechanism drives the screw shafts to rotate, the screw shafts change positions thereof relative to the nut members, thereby moving in the longitudinal direction. As a result, the upper rails supporting the screw shafts slide relative to the lower rails in the longitudinal direction, and thus a position of the vehicle seat, supported by the upper rails, is adjusted in the longitudinal direction.
The drive mechanism of the known power seat apparatus for the vehicle includes a motor and speed-reduction portions for reducing rotation speed of the motor and changing rotating directions of rotation axes. The speed-reduction portions are fixedly mounted to the upper rails disposed on the left side and the right side respectively, and the motor is coupled to either one of the speed-reduction portions. An output power of the motor is directly transmitted to an input gear of the speed-reduction portion to which the motor is coupled. To an input gear of the other speed-reduction portion, the output power is transmitted via a transmission shaft extending in a lateral direction of the vehicle. Each of the screw shafts is coupled to the output gear of the speed-reduction portion on the left side and the right side respectively by penetrating through the output gear respectively, and thus the output gears rotate the screw shafts.
In the speed-reduction portion of the above-mentioned drive mechanism, a worm including a rotation axis in the lateral direction serves as an input gear and is housed in an accommodating space formed in a housing. A worm wheel including a rotation axis in the longitudinal direction serves as an output gear and is housed in the accommodating space so as to mesh with the worm. The screw shaft is fixedly fitted in the output gear. In order to restrict the worm wheel from moving in an axial direction thereof, the accommodating space includes a stepped hole where the worm wheel is accommodated. That is, one of bearing portions each protrudingly provided on one of the end surfaces of the worm wheel is fitted into a bearing hole of the stepped hole, and thus the one of the end surfaces of the worm wheel contacts with a stepped portion of the stepped hole. The other one of the bearing portions is fitted into a bearing hole formed on the gear box cover, and thus the other one of the end surfaces of the worm wheel contacts with an end surface of the gear box cover. In this state, the gear box cover is fastened to the housing by means of screws.
According to the drive mechanism of the known power seat apparatus for the vehicle disclosed in JP2000-185419A, the gear box cover needs to be fastened to the housing by using plural screws in order to rotatably support the worm wheel serving as the output gear while restricting the worm wheel from moving in the axial direction thereof. Tightening plural screws requires time and effort, and an increased number of parts to be assembled, thereby increasing assembly complication and costs.
A need thus exists for a drive mechanism for a power seat apparatus for a vehicle, which is not susceptible to the drawback mentioned above.